This application is a continuation-in-part of application Ser. No. 10/224,949, filed Aug. 20, 2002 now abandoned, which is incorporated by reference herein in its entirety.
1. Field of the Invention
The present invention relates to a method for reducing shrinkage during sintering low-temperature-cofired ceramics by means of applying a constraining layer with windows on it. The present invention does not need to remove the constraining layer after firing so as to avoid possible damage to the surface flatness of ceramics in conventional technologies, and also simplifies the process procedures to lower the production cost. Therefore, multilayer ceramics with high quality is produced by the invention.
2. Description of the Related Art
Interconnect circuit boards are necessary for modern electronic products to meet the requirement of being light, thin and small. The interconnect circuit boards are either electronic circuits interconnecting each other electrically or mechanically, or subsystems from a number of extremely small circuit components combined in an arrangement. Such circuit components can be physically isolated and mounted adjacent to one another in a single interconnect circuit board to electrically connect to each other and/or extend from the interconnect circuit board thereby.
In the interconnect circuit board, complex electronic circuits generally require that several insulating dielectric layers be used to separate layers of conductors. The electrically conductive pathways interconnecting conductive layers through the dielectric are called vias. Such a multilayer structure allows a circuit to be more compact and occupy less space.
Among various methods for making a multilayer circuit, the method described in U.S. Pat. No. 4,654,095 is by co-firing a multiplicity of ceramic tape dielectrics on which heterogeneous materials such as resistors, capacitors, or conductors have been printed with metallized vias extending through the dielectric layers to interconnect various conductive layers. The dielectric layers are stacked in registry and pressed together at a proper temperature and pressure, and then fired to drive off the organics such as binders and plasticizers in the green ceramic body. All the ceramic and heterogeneous materials are sintered and densified thereby. This method has the advantages of performing firing only once, saving fabricating time and labor, and limiting the diffusion of mobile metals to prevent shorting between the conductive layers. However, because the sintering shrinkage behavior between the ceramics and heterogeneous materials is not the same, the firing condition is difficult to control. Furthermore, this X-Y dimensional uncertainty, resulting in misregistration during assembly of large and complex circuits, is particularly undesirable.
Nowadays, almost all shrinkages are controlled in the Z-direction to reduce X-Y dimensional shrinkage. Such processes are disclosed in U.S. Pat. No. 5,085,720 (E. I. Du Pont de Nemours and Company) and U.S. Pat. No. 5,130,067 (International Business Machines Corporation), which are incorporated herein as references.
In U.S. Pat. No. 5,085,720, the top and bottom of the green ceramic body each are applied with a release layer to form a “sandwich” structure. During burning out and sintering, a unidirectional pressure is applied to the surface of the release layer. The porosity of the release layer provides an escape pathway for the volatile components of the green ceramic body. Because the release layer does not shrink in firing, the X-Y shrinkage of the green ceramic body is reduced. However, the release layers cover both the top and bottom surfaces of the green ceramic body, and their removal should be performed after sintering for printing with and firing the conductors, resistors, and capacitors thereon. The cost of the method thus rises. In addition, the penetration of the inorganic binder contained in the release layer into the green body, and the contact angle, the viscosity, the porosity and the pore size of the release layer all affect the flatness of the ceramics surface during the removal of the release layer, and can easily make the circuit printing difficult and the products defective. When fabricating a large number of ceramic layers (e.g., more than 6 layers), middle layers of a dielectric body still shrink as the result of the forces not being distributed evenly by applying the release layers on the top and bottom of the green body (i.e., the forces on the top and the bottom and on the middle layers of the green body are substantially different).
U.S. Pat. No. 5,130,067 has described three approaches to reduce X-Y shrinkage of a dielectric body. With the first approach, constraint is applied to the outer edges of the dielectric body to provide an open escape path for volatiles and an entry path for oxygen. With the second approach, a co-extensive force is applied to the entire surface of the dielectric body by either using a co-extensive porous platen or by application of an air-bearing force to the surface. With the third approach, a frictional force is applied to the green body by use of a porous contact sheet which does not shrink or expand during firing. The contact sheet comprises a porous composition selected so that it remains porous during firing, does not fuse to the ceramics, is thermally stable and has continuous mechanical integrity/rigidity. The contact sheet maintains its dimensions during sintering and thus restricts the ceramic part from shrinking. After sintering, the contact sheet is removed by means of an appropriate removal procedure, such as polishing and scraping, which will not damage the surface of the ceramics. However, the three approaches described above have disadvantages. For example, the first approach easily leads to shrinkage of the ceramics and further affects shape of the circuit and flatness of the ceramics surface because it uses a weight produced by a fixture to reduce the shrinkage and the pressure distributes unevenly on the dielectric body. The second and third approaches need removal steps after firing, and the cost increases and the surfaces are damaged also.
In order to resolve the problems mentioned above, the present invention develops a novel method for reducing shrinkage during sintering low-temperature-cofired ceramics to save cost and improve the quality of the ceramic product.